I also need the lower section below the grill for a 1961 Perkins MF35 and have found the same conflicting descriptions. Here is what the AGCO online parts book says:
From the TO35 section:
186888M91 Grille Assembly, Lower (Gas) - Used Tractor Serial Number 204408 and Up.
180615M91 Grille Assembly, Lower (Gas) - Used Prior To Tractor Serial number 204408.
184988M91 Grille Assembly, Lower (Diesel)
From the MF35 section:
186888M1 Panel, Lower Grille
The MF35 section has the Perkins Diesel parts listings as well as the z134 gas and it only lists one lower grille panel. My guess is that someone at YT looked up the TO35 parts listing and saw that there was a specific panel listed for diesel and other panels showing 'Gas' but the TO35 diesel used the Standard Motors diesel, not the Perkins. So my guess is that after serial number 204408 the same lower panel will work with both the gas and Perkins diesel MF35s. I have not ordered one yet and this is just what I have come up with looking at the parts books. It would be nice to hear from someone with hands on experience with one of the new lower panels and a Perkins 35.